Yersinia pseudotuberculosis is a gram-negative, enteric bacterial pathogen that causes gastroenteritis, mesenteric lymphadenitis, which can be misdiagnosed as appendicitis leading to unnecessary surgery, and occasionally systemic disease in humans and other mammals. To cause these syndromes, the bacteria colonizes a variety of tissues in a mammal, including the lumen throughout the gastrointestinal tract (GI), the Peyers patches, mesenteric lymph nodes, and eventually the spleen and liver. Colonization of each of these tissues requires expression of at least one or more of the Yersinia virulence factors, called Yops. The Yops, which are found in all three pathogenic Yersinia spp. and share homology to virulence factors in other bacterial pathogens, are secreted into host cells via a type III secretion machinery where they disrupt properties of mammalian cells. Most Yops, studied to date, have several phenotypes in cells in culture and/or multiple protein targets in biochemical assays. The long-term goals of this project are to understand the bacterial factors needed to establish an infection in specific tissues and the host defense mechanisms targeted by bacterial virulence factors. The specific goals of this proposal are to characterize which Yops are important in the GI tract and lymph tissues, which cell culture and/or biochemical phenotypes of the required Yops are needed for colonization in the GI tract and lymph, and to characterize the host defense factors in the 01 tract that combat a Yersinia infection. This work has the potential to uncover heretofore-unknown aspects of host defense mechanisms in the GI tract. In addition, studies of yop mutant Yersinia strains in mouse strains with specific immune defects may indicate the facets of the immune system targeted by each Yop.